1. Field of the Invention
The present invention generally relates to an integrated type optical pickup and, more particularly, to an integrated type optical pickup having semiconductor lasers that projects laser beams having different wavelength.
2. Description of the Related Art
In Recent years, various kinds of optical discs have become popular as optical recording media. A compact disc (CD), a recordable compact disc (CD-R) and a rewritable compact disc (CD-RW) are classified into a CD group optical disc. A digital versatile disc (DVD), a recordable digital versatile disc (DVD-R), a rewritable digital versatile disc (DVD-RW) and S-DVD are classified into a DVD group high-density optical disc. Accordingly, it is preferable that a single recording and reproducing apparatus can record or reproduce a plurality of types of optical discs.
However, a laser beam having a wavelength of 780 nm is used for the CD group optical disc while a laser beam having a wavelength of 650 nm is used for the DVD group high-density optical disc. A beam spot of a laser beam having the wavelength of 780 nm cannot be reduced into a size equal to the size of each pit formed on the DVD group high-density optical disc. Accordingly, information recorded on the DVD group high-density optical disc cannot be read by the laser beam having the wavelength of 780 nm. On the other hand, a colorant used for the CD-R does not reflect a laser beam having the wavelength of 650 nm but let the laser beam passing therethrough. Accordingly, information recorded on the CD-R cannot be read by the laser beam having the wavelength of 650 nm.
Accordingly, in order to record or reproduce both the CD-R and the DVD group high-density optical discs, two semiconductor lasers that can generate laser beams having wavelengths of 780 nm and 650 nm must be used. When a single optical system is shared by two semiconductor lasers generating laser beams having the wavelength of 780 nm and 650 nm, the two light-emitting points of the laser beams must be as close as possible. Preferably, the distance between the two light-emitting points is less than 100 μm.
Accordingly, a semiconductor laser device has been suggested which has two horizontally arranged semiconductor lasers, one of which generates a laser beam having the wavelength of 650 nm and the other generates a laser beam having the wavelength of 780 nm. However, a characteristic of such an arrangement of the semiconductor laser chips is influenced by a width of each laser chip and a width of a mounting portion. Since a distance between the semiconductor laser chips must be as large as 300 μm to 400 μm, it is difficult to design an optical system of an optical pickup that requires the laser beams to be projected from a single light-emitting point or two approximated light-emitting points.
On the other hand, a semiconductor laser device having a single semiconductor laser chip has been suggested, which semiconductor laser chip can generate two laser beams having different wavelengths. Additionally, a method has been suggested in which two semiconductor laser chips are arranged side by side, each of which has a light-emitting point on an edge thereof. However, such a semiconductor laser device is not available since it is not placed on the general market.
Accordingly, a method has been suggested in which two semiconductor laser chips having a regular structure are used by approximating two light-emitting points in a pseudo manner by using reflection surfaces. That is, the two light-emitting points are arranged so as to be apparently very close to each other due to the laser beams being reflected by the reflecting surfaces.
Japanese Laid-Open Patent Application 11-39684 discloses a method in which light-emitting points are approximated with each other in a pseudo manner by using a mounting member having a triangular cross section.
FIG. 1 shows a laser chip mounting structure disclosed in Japanese Laid-Open Patent Application 11-39684. In FIG. 1, laser beams B1 and B2 are emitted from semiconductor lasers 2-1 and 2-1 mounted on a submount 4. The laser beams B1 and B2 are deflected by oblique reflection surfaces 6-1 and 6-2 of a triangular portion 6 formed on the submount 4, respectively, so that the light-emitting points from which the laser beams B1 and B2 are projected are apparently approximated with each other.
In order to achieve the a mounting structure shown in FIG. 1, the triangular portion 6 having a triangular cross section must be formed on the submount 4. There have been suggested some methods of forming the oblique reflection surfaces 6-1 and 6-2. However, it is difficult to make the oblique reflection surfaces 6-1 and 6-2 each of which forms an angle of 45 degrees with respect to the surface of the submount 4. Thus, those methods cannot be applied to a mass production process.
The mounting member having such a structure can be made by anisotropic etching of a silicon (Si) substrate. However, the silicon substrate cannot provide a large electric resistance, which is sufficient for electrically isolating two semiconductor laser chips from each other. Thus, the silicon substrate has not been used in actual products.
There is a method of forming a microprism on an insulating substrate, which can achieve the mounting structure shown in FIG. 1. That is, the triangular portion 6 is made by the microprism, which is mounted on the mounting member made of an insulating material. However, it is very difficult to form such a microprism having a triangular cross section, thereby increasing a manufacturing cost. A microprism having a triangular cross section can be formed separately from the mounting member so as to be placed on the mounting member made of an insulating material. However, in such a structure, there is a problem in that a distance between apparent light-emitting points is changed due to a change in a mounting height of the microprism.
FIGS. 2A and 2B are illustrations for explaining the change in the distance between apparent light-emitting points due to a change in the mounting height of the microprism. In FIGS. 2A and 2B, laser beams are projected from semiconductor lasers 10-1 and 10-2 along an optical axis 12 toward the microprism 14 while spreading in directions perpendicular to the optical axis 12. In order to prevent the laser beams from interfering with the surface of the mounting member 16, the microprism 14 is placed in a recess 16a formed in the mounting member 16.
In the structure shown in FIGS. 2A and 2B, if a depth of the recess 16a fluctuates, the position of the microprism 14 relative to the semiconductor laser chips 10-1 and 10-2 changes, as interpreted from comparison of FIG. 2A and FIG. 2B. Accordingly, the distance between the apparent light-emitting points of the semiconductor laser chip 10-1 and 10-2 is changed. Thus, the mounting member 16 including the recess 16a must be formed with a very high accuracy, thereby increasing a manufacturing cost of the mounting member 16. Thus, the mounting member 16 is not suitable for practical use.